The Boss has left the building.

The Al Horford era in Atlanta is over.

With it goes nine seasons of unprecedented success for the Hawks organization. Whether the run continues following several organization-altering moves over the past weeks remains to be seen. If so, it will be done without the player who has been a cornerstone of the organization for nearly a decade.

Horford agreed to a maximum deal of four years, $113 million with the Celtics on Saturday. Horford broke the news via social media as he posted on Twitter: “Celtic Pride!!!!!! with 18 shamrock emojis. The Hawks failed to offer the four-time All-Star center a maximum deal of five years, $153 million. After imposing a Saturday-evening goal to reach a deal, Horford will leave the team he joined as the No. 3 overall pick in the 2007 NBA Draft.

The Hawks’ top offer was reported by ESPN to be five years, $136 million. He left $23 million on the table. The Hawks’ agreement with Dwight Howard on a three-year deal Friday was at least in part an issue.

Horford cannot sign the deal until Thursday, after the end of the NBA’s moratorium period.

As negotiations between the Hawks and Horford continued all day Saturday, there were other suitors for Horford. The Celtics and Wizards were in the lead for his services, according to a person familiar with the situation. They were two of four teams that had meetings with Horford on Friday, the first day of the NBA free-agency period. The Rockets and Pistons also met with Horford.

There remained the possibility that Horford could return to the Hawks — even after the free-agent signings of Howard (three years, $70.5) and Kent Bazemore (four years, $70 million) — but it would have required the team to move about $18 million in salary.

The Hawks did make a “very significant” offer to Horford on Friday. However, based on the team’s current salary-cap situation at the time, he could not agree to it until they made roster moves. By Saturday morning, Horford informed the Hawks of his timeline to choose a team by the evening. The Hawks increased their offer during the day, but never reached the maximum or near-maximum deal he desired.

The Hawks had options to open salary-cap space to sign such a deal with Horford, a mainstay of the organization for the past nine seasons. While there are a multitude of ways it could be done, the bottom line is that it would require the team to move significant salary. They explored trades, including ones involving Paul Millsap.

With the NBA announcing the salary cap at $94.143 million Saturday, Horford’s max deal was four years, $113 million from another team and five years, $153 million from the Hawks. The latter is the same deal the Grizzlies gave Mike Conley, who has never been named to an All-Star team, on Friday.

Horford previously stated and stood by the stance before the free-agent signing period that he would like to remain in Atlanta. The only thing that could get in the way was a contract offer below what he believed he was worth. It didn’t come and Horford moves on to the team the Hawks defeated in the first round of the playoffs last season.

Bazemore spoke with Horford after he reached an agreement to re-sign with the team Friday to try to convince him to stay in Atlanta. Horford also reportedly spoke with Kyle Korver recently.

“We are so fortunate to work with a guy of the caliber of Al Horford,” Hawks general manager Wes Wilcox said last month. “We also believe in the quality of the relationships that we have built over the years. At the same time, we always respect a player’s right to choose, especially as an unrestricted free agent. But Al has been, is and will be a priority for the Hawks. I think we feel very good about (re-signing him).”

The Hawks have shown their preferred way to build the organization is to retain players and add through the draft. They re-signed all-star Millsap last season although were unable to retain DeMarre Carroll, who got a large deal from the Raptors.

Hawks coach Mike Budenholzer said following the season that Horford is “incredibly valuable” to the organization. The Hawks have the longest streak of consecutive playoff appearances in the Eastern Conference at nine — a run that started the year Horford was drafted.

Horford is the Hawks’ all-time leader in playoff games (74). He also ranks first in rebounds (621), second in points (958) and field goals (406) and fourth in assists (210). He is among the franchise leaders in several regular-season statistics including second in field-goal percentage (.535), sixth in blocks (698), eighth in rebounds (5,144) and 10th in player efficiency rating (19.1).

The Hawks sought to aggressively remake their roster after being swept by the Cavaliers in each of the past two postseasons, including the Eastern Conference finals two years ago.

The Hawks now move on to complete a roster that has been transformed with the loss of starting point guard Jeff Teague and Horford, the re-signing of Bazemore, the addition of Howard and two first-round draft picks. The Hawks still need at least a backup point guard. They have a multitude of future draft picks in their arsenal and newfound salary-cap space that that they can use.